Listeria monocytogenes is a common environmental Gram-positive bacterium that upon ingestion can cause the serious disease listeriosis (1). Listeriosis is normally contracted from ingestion of contaminated food by at risk populations which include the elderly, the immunocompromised and pregnant women (2-3). Disease symptoms can range from mild gastroenteritis to severe meningitis and spontaneous miscarriage (4). Current therapy calls for high dose aminopenicillins combined with gentamicin (5). Although L. monocytogenes is highly susceptible to this treatment in vitro, the fatality rate from confirmed cases of listeriosis remains high, sometimes reaching ˜30%, suggesting an increased need for better therapeutic strategies for treating listeriosis (6-7).
β-lactam antibiotics have been a critical part of treatment for Gram-positive bacterial infections since they were discovered (8). Unfortunately, due to the increasing frequency of antibiotic resistance, β-lactams are no longer effective against many pathogens, including certain penicillin resistant Streptococci and Enterococci, and most notoriously Methicillin-Resistant Staphylococcus aureus (MRSA) (9). MRSA strains, including the community associated strains such as USA300, contain the mecA gene which encodes the Penicillin Binding Protein 2A (PBP2A), a PBP that confers resistance to all approved β-lactams with the exception of ceftaroline (10-11). Similarly, resistance to drugs targeting Mycobacterium tuberculosis has been on the increase worldwide, motivating the search for new methods for identifying drug targets and understanding resistance mechanisms in Mycobacterium tuberculosis (see, e.g., Ioerger, Thomas R., et al., Identification of new drug targets and resistance mechanisms in Mycobacterium tuberculosis, PLOS One 8(9), September 2013, e75245, p. 1-13). The alarming increase in the development of antibiotic resistance, particularly to β-lactams, has resulted in a need for new strategies for antimicrobial therapy.
Despite this widely acknowledged need, truly novel antibiotic classes have not been developed for decades. Collectively, the cell wall active β-lactams (various penicillins, cephalosporins, monolactams, and carbapenems) have been the most prescribed antibiotics worldwide since penicillin was discovered and remain so today, despite increasing resistance.
S. aureus, and many other important pathogens, including L. monocytogenes, Mycobacterium tuberculosis and Enterococcus faecalis, express a bipartite membrane-associated eukaryotic-like serine/threonine kinase that has one or more extracellular repeat of a homologous family of PBPs (12). This family of proteins is known as the Penicillin binding protein and Serine-Threonine kinase associated protein (PASTA) kinases (12). PASTA kinases have extracellular penicillin binding domains that have previously been shown to bind fragments of peptidoglycan, likely generated by cell wall damage or remodeling, and an intracellular serine threonine kinase domain, similar to those found in eukaryotic cells (13).
While the substrates and function of the PASTA kinases are incompletely defined, they appear to have varied functions in different organisms ranging from playing a role in biofilm formation (Streptococcus mutans) to being essential in some organisms (M. tuberculosis) (14-15). Deletion of Stk1, the PASTA kinase in S. aureus, reverses the methicillin resistant phenotype in MRSA (16-17). In addition, deletion of the PASTA kinase in E. faecalis, PrkC, led to a >100-fold sensitization to certain β-lactam antibiotics (18). However, the mechanism for this effect remains unclear.
Rajagapol et al. (WO 2013/066469) disclose the use of certain kinase inhibitors to increase the sensitivity of bacterial pathogens to β-lactam antibiotics. As is well-known in the art, kinase inhibitors encompass a large family of compounds that vary substantially in structure, specific target, and targeting mechanism, and the successful use of a specific kinase inhibitor for a given therapeutic purpose can not be predictably applied to other kinase inhibitors. This is particularly true for inhibitors having a substantially different core structure than those that have been previously disclosed. Thus, there is a need in the art for other compounds that can be used to increase the susceptibility of bacterial pathogens to β-lactam antibiotics.